1. Field of the Invention
The present invention relates to a toner for developing electrostatic images to be used in electrophotographic apparatuses that utilize the electrophotographic process, such as copying machines, printers, facsimiles, and the like, and a manufacturing method thereof, a developer for electrostatic images using the toner for developing electrostatic images, and an image forming method utilizing the toner for developing electrostatic images.
2. Description of the Related Art
Methods for visualizing image information through an electrostatic latent image, such as the electrophotographic method, or the like, are widely used in various fields at present. With the electrophotographic method, an electrostatic latent image on the surface of a photosensitive material is developed through a charging step, an exposure process, and the like, and the electrostatic latent image is visualized through a transfer step, a fixing step, and the like.
A number of methods are known as electrophotographic methods. In general, a latent image is formed electrically by one of various means on the surface of a photorecepter (latent image holding member) which utilizes a photoconductive substance. The formed latent image is developed with a toner to form a toner image. Thereafter, the toner image on the surface of the photorecepter is transferred onto the surface of an image receiving material such as paper or the like optionally via an intermediate transfer material. The transferred image is subjected to a fixing process such as heating, pressurizing, heat-pressurizing, or the like, such that a fixed image is formed. Toner which remains on the surface of the photorecepter is cleaned by various methods as necessary and may be utilized for development of a toner image again, as required.
As a fixing technique for fixing a transferred image which has been transferred onto the surface of an image receiving material, a heat roll fixing method is used generally. In this method, an image receiving material having a toner image transferred thereto is inserted and fixed between a pair of rolls (a heat roll and a pressure roll).
Generally, a polyester resin or a vinyl polymer using polystyrene as its base is commonly used as an electrophotographic toner material from the viewpoint of its antistatic characteristics, resin strength and coloring property upon being mixed with a colorant. Particularly, the polyester resin among these toner materials is produced from a polyvalent carboxylic acid and a polyhydric alcohol by a dehydration or ester exchange in the presence of a polycondensation catalyst. In the production process, an organic tin catalyst has been usually used as the polycondensation catalyst from the viewpoint of, for example, its polymerizing speed and generation of byproducts. However, in recent years, a lot of discussion has arisen as to the effects of residual organic tin on the environment and the human body. Therefore, energetic studies are conducted concerning the shift from the organic tin catalyst to a catalyst containing an element other than tin such as titanium, antimony or aluminum (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-302854 and 2000-284538).
However, electrophotographic toners produced using these catalysts other than tin have various problems which should be solved to enable practical use; for example, charging faults due to a residual catalyst or side-reactions, reduced color-developing characteristics due to coloring of a resin and limitation on industrially polymerizable polyester monomers (polymerization reactivity) and particularly, a difficulty in the use of propylene oxide adducts of bisphenol A which are necessary to attain a good balance between sufficient strength and antistatic characteristics required for electrophotographic toners.
On the other hand, with increased demand for saving the power required for image formation in recent years, the technological development of a so-called low-temperature fixing toner which aims at saving the electric power consumed in the fixing process, which is one of the processes consuming the highest amount of energy in the electrophotographic process, has been actively promoted.
In the low-temperature fixing toner, a lowered fixing temperature of the toner generally involves simultaneous decrease in the glass transition point of the toner, which makes it difficult to achieve all of the lowered fixing temperature, the desired toner shelflife, and the desired preservability (offset resistance) of the final output image obtained. Therefore, in order to achieve both of the low-temperature fixability and the desired toner shelflife, the toner is required to have so-called sharp melting characteristics, which are such characteristics that the viscosity of the toner is decreased sharply in the vicinity of the fixing temperature while the glass transition point of the toner is maintained high.
As a promising approach to the above technical problems, a method has been proposed in which a crystalline resin having sharp melting property is used as a binder resin.
In an electrophotographic full-color toner, a polyester resin has been conventionally used as the binder resin because of its superior coloring property and adhesiveness to paper. Therefore, energetic studies are conducted with respect to crystalline polyesters as the crystalline resin having sharp melting property.
Also, along with a demand for high image quality in current electrophotographic technologies, there have been many studies on reduction of the size of particles in a method of producing toner chemically (e.g., dissolution suspension, emulsion-polymerization aggregation, or suspension polymerization). Studies have been also conducted on the combination of these crystalline resins and low-temperature fixing techniques. However, these crystalline resins, especially, crystalline polyesters have the following problems concerning the electric characteristics and image characteristics of the resins when applied to electrophotographic toner. These problems are obstacles to the practical use of these resins.
When a crystalline resin is applied to an electrophotographic toner so as to achieve low-temperature fixability, the melting point of the resin is an important factor in the selection of the resin. In general, when the electrophotographic toner aims at low-temperature fixing, it is important to select a material which melts sharply at a temperature as low as possible, namely a material having a low melting point. It is also important for the toner to have storage stability required for the toner and post-fixing image blocking characteristics. Therefore, crystalline resins having a melting point of about 80° C. are widely studied at present. However, these crystalline resins having low melting points have electric resistances as low as about 1/100 to 1/1000 the electric resistance of a resin usable for usual electrophotographic toner. When these crystalline resins are blended as a toner component in a toner, the charge of the toner is small and also, the electrified charge gradually leaks with time; therefore, big practical problems of charging inferiors due to low electrification and the leakage of charge accompany the resultant electrophotographic toner utilizing electrification by friction as its basic principle.
Also, the aforementioned crystalline resin having a melting point of about 80° C. has a skeleton mainly composed of a long alkyl chain as its resin skeleton and is therefore fragile and has poor toughness. Therefore, the use of the crystalline toner as electrophotographic toner easily poses the problem concerning collapse of the toner in a machine due to a lack of resin strength and cleaning inferior caused by filming on the photoreceptor, thereby causing defects in final images.
To deal with these problems, studies as to a polymer blend method have been energetically made in recent years in which a crystalline resin mixed with a non-crystalline resin that is currently used as an electrophotographic toner material is used as electrophotographic toner. In this method, important technical requirements for the polymer blend are that the resins to be mixed have moderate compatibility with each other and preferably have a UCST (Upper Critical Solution Temperature) representing the so-called semi-compatibility. Since the crystalline resin to be used has a structure based on an alkyl chain as described above, the structure has very low polarity and therefore the so-called resin structure has a low SP value.
Accordingly, the non-crystalline resin to be used has to be designed to have a low SP value from the viewpoint of its compatibility. As the non-crystalline resin having a low SP value to be used as an electrophotographic toner material, a resin having a skeleton including a propylene oxide adduct of bisphenol A will become more important. However, when the propylene oxide adduct of bisphenol A is used in the future, the problem as to its production and technical problems accompanying its application to an electrophotographic toner described above should be solved.
As explained above, in the technical prospects of the electrophotographic toner, there is a large problem concerning how to produce a polyester resin by polymerizing a monomer component of a propylene oxide adduct of bisphenol A in the presence of a tin-free catalyst without impairing image characteristics such as charging characteristics and coloring property so as to put the polyester resin in practical use.